Mega Drive / Genesis - RGB caps and resistors revisited

[MtothaJ]

@MtothaJ
I've tried the completely unmodded system, which seemed fairly stable, but had some slight artifacts in the display. I thought 'maybe these are due to the MD1's csync being from pin 10 rather than 11' and so put a straight wire from the encoder pin 11 to the csync pin on the DIN connector (I figure I don't need the 200uF cap/75ohm resistor here, if it's in the cable). Now the image is very jumpy, and blanks out constantly. On both setups, colours have looked pretty much fine.

Don't have any knowledge regarding the VGA stuff unfortunately. If you are using cysnc you definietly need the 220uf (or 470uf depending on which encoder you have) and 75ohm resistor combo on the csync line - but I understand that you already have these on your cable so they should not be duplicated inside the machine. After the mods do you get a good RGB picture when plugging into a scart TV or pro monitor? If so, this would indicate that that side of things is fine and its the VGA part that may be the culprit.

[fluxcore]

Sadly I have no such monitor, hence the need for the adaptor Also I don't yet have anything else to hook up to it that is known working, to confirm a fault in the adaptor. Thanks though.

Which encoder requires which capacitor? I don't believe I've seen mention of that.

Cheers

[DemVil12]

One question - is the official MK-1637-09 cable aka "Adapteur R.V.B" for MD2 worth anything? Or a decent modern handmade equivalent will outperform it ? I can get one for about 30$ with shipping.

One more question - as i can understand from Google, it's a 12V cable. If i connect this one to a modern lcd tv "as is", i will not burn anything?
Or i need to disable that upconverting power chip, as supposed in one of the guides ?

[MtothaJ]

One question - is the official MK-1637-09 cable aka "Adapteur R.V.B" for MD2 worth anything? Or a decent modern handmade equivalent will outperform it ? I can get one for about 30$ with shipping.

One more question - as i can understand from Google, it's a 12V cable. If i connect this one to a modern lcd tv "as is", i will not burn anything?
Or i need to disable that upconverting power chip, as supposed in one of the guides ?

The upconverting 12V power chip thing is basically so that the TV switches to 4:3 mode when you power on. THis is a welcome feature if you are using a widescreen TV since on all of the homemade / unofficial cables it would switch to 16:9 mode and require readjustment everytime you power off /on to e.g. change a game. Definietly safe and no need to disconnect anything.
At the moment I am using the official cable with a Sony TV and I have the homemade cable with the big caps and pots connected to the BVM monitor. My impression is that the official cable gives a brighter picture than the homemade one, even though I have set the pots to 75ohms using a multimeter. I think that wherever possible its nearly always better to go with the official cables.

[fluxcore]

Don't have any knowledge regarding the VGA stuff unfortunately. If you are using cysnc you definietly need the 220uf (or 470uf depending on which encoder you have) and 75ohm resistor combo on the csync line - but I understand that you already have these on your cable so they should not be duplicated inside the machine. After the mods do you get a good RGB picture when plugging into a scart TV or pro monitor? If so, this would indicate that that side of things is fine and its the VGA part that may be the culprit.

Hah! The problem came down to the power supply for the RGB->VGA board (GBS-8200), which I had already ruled out as being ok. I was having sync issues, and read that the power supply could cause these. I tried using a PC power supply for the juice, but the problem remained, so I assumed my 2.8A wall wart power adaptor was therefore fine.

Strangely, with the PAL MD1 in 50Hz mode, it was much more stable an image than in 60Hz (but still bad). Perhaps the converter requires less power then, or something.

When I recently obtained a NeoGeo MVS (which I expected less sync issues with, but had the same symptoms), I noticed that when the screen was shaking, the power LED was flickering, indicating a low power problem. Tried out the PC PSU again, and everything was then fine! And now that I've performed the csync mod on the MD1, it works too.

So yeah, I guess the lesson is not to rule out the power supply just because swapping it doesn't fix your current problem

[zyrobs]

The TV standard is not relevant since we are talking about RGB connection.
French Mega Drive actually does not output anything other then RGB - no RF lead socket / no composite.
It uses csync instead of composite for sync but the cable can eb modded to enable both composite and csync:
http://www.segakore.fr/articles/mod_...rgb_fr_md.html

This is very interesting. Now I have to pick up a French model.

[MtothaJ]

This is very interesting. Now I have to pick up a French model.

I think there's no need to specifically pick up a French model - any other MD will be just as good provided that you have the Csync line wired to the csync pin in the AV Out socket.

[MtothaJ]

Recieved this yesterday and have been testing all evening. So far the results are excellent - no inteference, no colour shifts etc. Picture brightness is simply ideal also.
The cable fired up straight away on my machine, it takes it sync from csync, which I have wired internally to csync out and not csync in on the encoder, but my feeling is that this wouldn't make much difference (can test this since I have another unmodded machine if anybody is interested).

Just to confirm I have checked on the unmodded machine and all you get is a black screen - so basically the csync mod is a prerequisite for anyone wanting to use this cable.

[MtothaJ]

I wonder at this point, if something is up with the monitor? Cause with my PVM wired up the way I originally stated, I have zero issues at all. So this all makes me so curious as to what's going on now.

I think that you might have been right all along - I tested the monitor more thouroughly with a RGB DVD player and also get the colour shift problem. So when I get a moment I will be sending the monitor off for repair. I think the problem is relatively minor that said given the fine geometry etc. of this particular unit I will probably take the opportunity and order a recap and professional calibration while I am at it since I plan on keeping this unit indefinietly. WIll advise on costs etc. once I get around to it.

[Bibin]

I didn't know a Mega Drive could cause anyone so much RGB trouble... I just have 75 ohm resistors in series with the RGB lines, and use composite video as sync on all my PVMs. Never had a single problem this way - everything is clear and balanced. This same setup works on my OSD-hack RGB TV and numerous projectors that take 15khz RGB.

[MtothaJ]

I think that these problems can be attributed to the faulty monitor. I have hooked up the Megadrive via Scart RGB to an LCD TV and am now using a 21" CRT Sony Trinitron and it worked fine with resistors alone on the RGB lines.